Personally prefer Vegas. It had a rough start, but patches fixed just about every glitch. The game itself is an overall upgrade to 3. Fallout 3 had a more immersive world, while I found Vegas to just be more fun. So, up to you.

get F3 off TPB and DLC if you want to and rather buy fallout new vegas game of the year through steam, that way you can quite easily mod it through nexus mod manager

Fallout 3 got the whole Fallout as in quite good
while F:NV is more straight or shall we say swinging fun.
just never start F:NV before F3 if you plan to play F3. It's like the difference between Oblivion and skyrim (I'm only countin diffrence here people)